Fire that generates from world generation (such as patches of fire in the Nether) will never receive block updates and thus will never be able to spread. This is most obvious in the Nether.
To reproduce, place tons of flammable material around a patch of natural Nether fire. The fire will never spread to any of those flammable blocks.
However, when the player places the fire and then surrounds that fire with flammable material, the fire will spread.
Linked issues
Attachments
Comments 4
Note: If this gets fixed, there will be a MASSIVE performance hit upon generating nether chunks. There may also be lasting issues because of firetick.
Confirmed, dripleaf/carpet burn really fast so it's pretty easy to see the difference with those blocks, especially when you crank random tick speed up